1. Field of the Invention
The present invention relates to a fuel cell combined with an improved hydrogen gas sensor. In particular, the improved hydrogen gas sensor may be employed to measure a hydrogen concentration in an anode exhaust passage within an electrochemical fuel cell.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions.
The fuel fluid stream supplied to a fuel cell anode typically comprises hydrogen, which may be, for example, substantially pure hydrogen, or a dilute hydrogen stream such as a reformate stream. Further, the hydrogen exhaust, or a portion thereof, may be recirculated back to the fuel cell. The oxidant fluid stream supplied to a fuel cell cathode typically comprises oxygen, which may be, for example, substantially pure oxygen, or a dilute oxygen stream such as air.
At the cathode, oxygen in the air is consumed by the electrochemical process and a small amount of oxygen diffuses through the membrane to the anode. The oxygen concentration at the cathode flow field inlet is about 21% and may typically be about 10% at the oxidant flow field outlet. The average concentration of nitrogen from the inlet to the outlet of the cathode flow field is thus about 85%. The nitrogen diffuses through the membrane to the anode and if allowed to equilibrate, the nitrogen concentration at the anode would be essentially the same as at the cathode, about 80–85%. In operation, this results in nitrogen concentration in the recycle loop increasing until the partial pressure of nitrogen is nearly the same on both the anode and cathode sides of the membrane. U.S. Pat. No. 6,541,141 discloses periodically, for example every five minutes, purging the anode side to prevent the build-up of nitrogen gas.
In a fuel cell, gas sensors, such as hydrogen sensors may be used to monitor the hydrogen concentration in the fuel streams. Instead of simply periodically purging the anode exhaust stream periodically, the anode exhaust could be purged as a result of a measured hydrogen concentration falling below a threshold value. Further, hydrogen concentration may be used as an indicator of the fuel cell performance and operating efficiency. For example, if there is an excessive amount of hydrogen in the fuel stream exhausted from the fuel cell, it may indicate poor operating efficiency. U.S. Patent Application No. 2002/0110713(now U.S. Pat. No. 6,852,434), which is hereby incorporated by reference in its entirety, discloses the use of a gas sensor in the interior fluid passages within a fuel cell assembly or within fluid passages employed to transport reactant fluid streams to or from the fuel cell(s).
The present fuel cell assembly incorporates an improved hydrogen sensor that operates reliably and accurately when located in a fuel fluid stream passage within an electrochemical fuel cell.